I know I didn't write out the explanation for it on the character sheet, but here it is: His mother, being a model, is good with cosmetic magic (not that she uses that around the Muggles; it's just something she picked up when she was, herself, a student, being somewhat vain and obsessed with looking good). So, basically, one day, he just asked her to change his hair color, after seeing her do it at home for the millionth time. And every so often, she just had to re-cast the spell on him.
And as play begins, he'll know the spell, just from having it cast on him multiple times over the past few years. And once he's got his wand, he'll be able to do it himself.